The present invention relates to a process for preparing dibenzyl carbonates by reacting unsubstituted dialkyl carbonates with optionally substituted benzyl alcohols in the presence of a catalyst.
Dibenzyl carbonates are required as precursors for benzyl carbazates. Benzyl carbazates for their part are used as intermediates for preparing crop protection agents and pharmaceutics. In particular, they are used for peptide syntheses (see EP-A 106 282). According to J. Biol. Chem. 266, 5525 (1991), hydrazinosuccinate, an inhibitor of aspartate aminotransferase, can be prepared from benzyl carbazate. EP-A 143 078 describes the use of benzyl carbazate for preparing crop protection agents.
It is known that symmetric dibenzyl carbonates can be prepared by phosgenation (see J. Am. Chem. Soc. 70, 1181 (1948)). This process has the disadvantages that the product is obtained in highly impure form and that the by-product benzyl chloride is always present. Furthermore, this procedure is not particularly economical since the alcohol by-product has to be either disposed of or recovered by an additional isolation step. Finally, the handling of phosgene requires particular technical safety arrangements.
Accordingly, there continues to be a demand for a favourable process for preparing dibenzyl carbonates.
The present invention, accordingly, provides a process for preparing dibenzyl carbonates of the formula 
in which
R in each case represents C1-C6-alkyl, C6-C12-aryl or halogen and
n in each case represents zero or an integer from 1 to 5,
characterized in that a dialkyl carbonate of the formula 
xe2x80x83in which
Rxe2x80x2 in each case represents a C1-C4-alkyl radical,
is reacted with a benzyl alcohol of the formula 
xe2x80x83in which
R and n are as defined under formula (1),
in the presence of a basic catalyst, and the highly volatile components and the catalyst are removed from the reaction mixture.
The two benzyl groups in formula (I) are generally identical. If a plurality of radicals R is present in a benzyl group in the formulae (I) and (III), these can be identical or different.
In the formulae (I) and (III) R preferably represents C1-C4-alkyl, phenyl, fluorine or chlorine and n preferably represents zero, 1 or 2.
Particularly preferably, n represents zero, i.e., according to the invention, particular preference is given to preparing unsubstituted dibenzyl carbonate from a dialkyl carbonate of the formula (II) and unsubstituted benzyl alcohol.
In formula (II) the two radicals Rxe2x80x2 are generally identical. Rxe2x80x2 preferably represents methyl or ethyl.
Suitable catalysts for use in the process according to the invention are the most varied basic compounds. For reasons of easier separability, after the process according to the invention has ended, solid basic compounds are preferred for use as catalyst. Examples are: alkali metal hydroxides, such as lithium hydroxide, sodium hydroxide and potassium hydroxide, alkaline earth metal hydroxides, such as calcium hydroxide, alkali metal carbonates and bicarbonates and alkaline earth metal carbonates and bicarbonates, such as lithium carbonate, sodium carbonate, potassium carbonate, calcium carbonate, sodium bicarbonate and potassium bicarbonate, and alkali metal alkoxides or alkaline earth metal alkoxides, such as lithium methoxide and sodium methoxide. When using alkali metal alkoxides and alkaline earth metal alkoxides, preference is given to those which are derived from the benzyl alcohol employed in each case or from the alcohol on which the dialkyl carbonate employed is based (for example, sodium methoxide is preferred if the dialkyl carbonate used is dimethyl carbonate), in order to avoid the formation of undesirable by-products.
It is also possible to use, as catalyst, amino compounds having relatively high molecular weights of, for example, from 100 to 200. Examples of these are bicyclic amino compounds, such as 1,5-diazabicyclo[4.3.0]non-5-ene and 1,8-diazabicyclo-[5.4.0]undec-7-ene. It is also possible to use basic titanium compounds, such as titanium(IV) isopropoxide, and basic tin compounds, such as dibutyltin oxide and dimethyltin didodecanate.
Preferred catalysts are:
sodium carbonate, potassium carbonate, sodium hydroxide and potassium hydroxide
The catalyst can be employed, for example, in amounts of from 0.0001 to 20 mol %, based on the dialkyl carbonate used.
For carrying out the process according to the invention, the dialkyl carbonate of the formula (II) in question and the benzyl alcohol of the formula (III) in question and the catalyst can be reacted, for example, in a molar ratio of 1: (20 to 1.6):(0.1 to 0.0001), preferably 1: (8 to 1.8):(0.01 to 0.0003) and particularly preferably 1: (5 to 2):(0.01 to 0.001). If appropriate, the reaction can be carried out in the presence of a solvent which is stable under the reaction conditions, for example in the presence of aromatic hydrocarbons or cycloaliphatic hydrocarbons. The process is preferably carried out in the presence of toluene.
The process according to the invention can, for example, be carried out at temperatures of from 50 to 150xc2x0 C., preferably from 60 to 160xc2x0 C., and at a pressure of, for example, from 1 to 0.001 bar.
The alcohol (=Rxe2x80x2OH) formed during the course of the process according to the invention is preferably distilled off even during the reaction. This process can be promoted, if appropriate, by addition of an azeotrope former, for example an aromatic hydrocarbon, in particular toluene.
After the reaction according to the invention has ended, the reaction mixture can be worked up, for example, by removing any highly volatile components which may still be present, for example alcohol Rxe2x80x2OH, for example by distillation, if appropriate under reduced pressure, and then removing the catalyst, for example by filtration or by extraction with water. Any excess starting material which may be present, for example excess benzyl alcohol of the formula (III), can be removed, if appropriate, by distillation. The reaction product obtainable in this manner can, if appropriate, be purified further with the aid of known methods, for example by (high-vacuum) distillation or (low-temperature) crystallization.
In an exemplary embodiment of the process according to the invention, the starting materials of the formulae (II) and (III) are initially charged together with the catalyst and, if appropriate, a solvent or azeotrope former, the mixture is heated to reaction temperature and the alcohol Rxe2x80x2OH formed is removed even during the reaction, for example by distillation, if appropriate under reduced pressure and/or if appropriate together with an azeotrope former. Remaining alcohol Rxe2x80x2OH and, if appropriate, any solvent or azeotrope former which is still present can be removed after the reaction has ended, for example at from 100 to 160xc2x0 C. and with lowering the pressure, for example to down to 0.001 bar. Afterwards, but of course also directly after the reaction has ended, the catalyst can be removed; in the case of a solid catalyst, for example, by filtration or decantation, otherwise, for example, by extraction with water. What remains is the dibenzyl carbonate of the formula (I) prepared, generally in purities of more than 90%, frequently more than 95%.
If this procedure is followed, the last amounts of Rxe2x80x2OH removed by distillation generally contain amounts of unreacted dialkyl carbonate of the formula (II). Such fractions can advantageously be added to the next reaction batch.
It is extremely surprising that, using the process according to the invention, it is possible to prepare dibenzyl carbonates of the formula (I) in high purity and in a technically simple manner. The alcohol Rxe2x80x2OH, which is formed as by-product in the process according to the invention, can be recycled into the preparation of the dialkyl carbonate of the formula (II) to be used as starting material. The process according to the invention does not require particular technical safety arrangements.
The invention is further described in the following illustrative examples in which all parts and percentages are by weight unless otherwise indicated.